An electrosurgical unit includes a radio frequency generator and its controls, which can be used for cutting or coagulating with high frequency electrical energy such as pulses shaped to enhance cutting or coagulation. Using an electrosurgical generator in a surgical procedure, it is possible for the surgeon to cut, to blend or cut with hemostasis, or to purely coagulate. The surgeon can easily select and change the different modes of operation as the surgical procedure progresses. In each mode of operation, it is important to regulate the electrical power delivered to the patient to achieve the desired surgical effect. Applying more power than necessary results in tissue destruction and prolongs healing. Applying less than the desired amount of electrical power inhibits the surgical procedure. It is desirable to control the output energy from the electrosurgical generator for the type of tissue being treated. Different types of tissues will be encountered as the surgical procedure progresses and each unique tissue requires more or less power as a function of frequently changing tissue impedance. Even the same tissue will present a different load as the tissue is desiccated and the position and size of the electrosurgical tool will effect the load. That is, the deeper the tool is moved into the tissue or the further the tool is pulled from the tissue will change the impedance or load. Accordingly, all successful types of electrosurgical generators use some form of automatic power regulation to control the electrosurgical effects desired by the surgeon.
Two conventional types of power regulation are in commercial electrosurgical generators. The most common type controls the DC power supply of the generator by limiting the amount of power provided from the AC mains to which the generator is connected. A feedback control loop compares the output voltage supplied by the power supply to a desired setting to achieve regulation. Another type of power regulation in commercial electrosurgical generators controls the gain of the high-frequency or radio frequency amplifier. An analogue feedback control loop compares the output power supplied from the RF amplifier for adjustment to a desired power level. The output is adjusted accordingly but generators commonly and currently in use do not digitally measure RF output power delivered to the load and thereafter regulate accordingly. Usually, the generators are run open loop, i.e. without feedback but if controlled, then only to a constant radio frequency output voltage.
Specifically, U.S. Pat. Nos. 3,964,487; 3,980,085; 4,188,927 and 4,092,986 have circuitry to reduce the output current in accordance with increasing load impedance. In those patents constant voltage output is maintained and the current is decreased with increasing load impedance. Similarly, U.S. Pat. No. 4,126,137 controls the power amplifier of the electrosurgical unit in accord with a non linear compensation circuit applied to a feedback signal derived from a comparison of the power level reference signal and the mathematical product of two signals including sensed current and voltage in the unit.
Known types of radio frequency power regulation have achieved moderate success but certain undesirable characteristics are associated with each. One undesirable characteristic involves the response time for regulation. The impedance of the different tissues encountered during the surgical procedure can fluctuate substantially. In moving from a high impedance tissue to a low impedance tissue, the low impedance tissue may be needless destroyed or damaged before the electrosurgical generator can reduce its output power to a level compatible with the lower impedance of the tissue. Similarly, when a high impedance tissue is encountered, the output power from the generator may be momentarily inadequate to create or continue the precise surgical effect desired by the surgeon. Wherefore, execution of the surgical procedure becomes difficult or impossible. Recognizing this problem is U.S. Pat. No. 4,658,819 wherein the power delivered to the load is a function of the voltage from a DC supply and the load as measured by sensors of load voltage and current. A microprocessor controller digitizes the sensing signals and computes the load impedance and actual power being delivered to the load. The microprocessor controller accordingly repeats the measurement, calculation and correction process approximately every 20 milliseconds as long as the generator is operating.
Another problem of radio frequency output power regulation in previous electrosurgical generators results because they have been designed to attain maximum power transfer at intermediate impedance ranges. As with amplifiers, an electrosurgical generator will achieve maximum power transfer when its internal impedance equals the output load impedance to which it is connected. At high impedances, the power delivered typically rolls off because of the difference between load impedance compared and the internal impedance. To compensate, surgeons increase the initial power setting to a level higher than necessary. Once the incision passes through the high impedance tissue, the output power setting remains too great and tissue destruction or undesirable surgical effects result. For example, the initial incision passes through skin with a relatively large percentage of dead cells, which contain considerably less moisture than other cells in tissues beneath the skin; that is, the epidermis has increased impedance compared to the impedance of the tissues therebelow. A higher power setting is required for the initial incision and thereafter a reduced amount of power will work. With commercially available electrosurgical generators, the initial incision is often deeper than desired because the active electrode, i.e., the electrosurgical instrument, cuts deeper than the surgeon desires due to the excessive energy delivery. The surgeon desires to control the depth of the incision and conduct the surgical procedure in controlled depth levels. If the power regulation is greater than needed, a deeper incision in certain areas results in undesired bleeding. For that reason most surgeons prefer to make the initial incision using a conventional scalpel, instead of using the active electrode blade of an electrosurgical generator.
Another radio frequency output power regulation related problem of available electrosurgical generators is open circuit flashing just prior to the start of the surgery. Prior to the electrosurgical procedure commencement, no output power is supplied due to the open circuit condition. The regulation circuit attempts to compensate with maximum power delivery. When the active electrode is positioned an operative distance from the tissue, an arc of relatively high voltage ensues due to the maximum power delivery capability initiated by the power regulation circuit. Continual arcing is desired in the coagulation (fulguration) mode of operation but is otherwise undesirable. The power regulation circuit eventually reduces the excessive power but the initial arcing or flash may already have caused excessive tissue destruction. The flash and excessive tissue destruction can occur anytime the surgeon moves the active electrode toward the tissue.
Open circuit or excessively high output impedance conditions increase the risks of alternate path burns to the patient. Alternate path burns occur when current flowing from the patient to some surrounding grounded conductive object, such as the surgical table, rather than returning to the electrosurgical generator through the patient return electrode. Reducing the output voltage under open circuit or high impedance conditions reduces the magnitude of and potential for radio frequency leakage currents.
Another radio frequency output power regulation related problem of commercial electrosurgical generators relates to shorting the output terminals of the generator. A frequent though not recommended, technique of quickly determining whether an electrosurgical generator is operating is to simply short the two output electrodes and observe an electrical spark. A possible result of shorting is the destruction of the power supply in the generator. The generator quickly attempts to regulate from a high power open circuit condition to a short circuit low impedance condition. Due to the limitations on regulating speed, the electrical power components of the power supply are overdriven and quickly destroyed before adequate compensation can occur.
U.S. Pat. No. 4,727,874 discloses an electrosurgical generator with a high frequency pulse width modulated feedback power control wherein each cycle of the generator is regulated in power content by modulating the width of the driving energy pulses. Instantaneous analysis of parts of the high frequency signals of the effects of impedance loads on the electrosurgical unit in real time is not possible. It is desirable to be able to examine a series of RF pulses and control the output with respect to the real time effect on tissue. Instantaneous corrections to the output are not possible; only changes over the average of the output pulses are feasible, see for example U.S. Pat. No. 4,372,315. That patent discloses a circuit which measures impedances after delivering a set number of radio frequency pulses on a pulse burst by pulse burst basis. U.S. Pat. No. 4,321,926 has a feedback system to control dosage but the impedance sensing is not on a real time basis.
Electrosurgical medical procedures require controllable and close regulation of the cutting and/or coagulating high frequency energy. The energy application must be limited to a desired surgical area in order that no damage be sustained by important structures or organs in the immediate vicinity of the cutting or coagulation. Whether cutting or coagulating, the tissue is supplied with monopolar electrosurgical energy. The tissue acts as a load which in electrical terms is considered as a variable impedance that is a function of the nature of the tissue being surgically treated. The load impedance has resistive, capacitive and inductive components and the energy pathways from the electrosurgical unit to the tissue similarly add resistive, capacitive and inductive components.
It would be preferred to instantaneously measure the variations of resistance, inductance and capacitance and correct the output of the electrosurgical unit accordingly. This, however, is impossible to do but output parameters such as voltage, current and power of the electrosurgical unit may be measured and/or calculated. Similarly, selected operational parameters such as constant current, constant voltage, and constant power can be regulated but not on an instantaneous level since the frequency of the pulses is typically 500 kilohertz. Circuits commonly in use for controlling the output of an electrosurgical unit are incapable of the response times necessary.
Analog measurement of output signals from instruments such as the electrosurgical unit are well known and in use because the physical world is primarily analog and the processing of analog signals in electronic circuits is well known and accomplished easily. For example, amplification, filtering, frequency modulation, and the like are common electronic functions of circuit designed to handle analog signals. Such signals tend to be continuous and therefore detectors of analog signals have difficulty in recognizing discontinuities in the signal brought about by change.
Digital or discreet signals are those that change from one condition to another distinct condition. For example, an "on" or an "off" condition is easily measured since there is no continuity in the change from "on" to "off". The advantage in having to deal with only two conditions, i.e. the existence of either one or the other, limits measurement and has a definite benefit since no subjective interpretation need be applied. Numerous gains are available with digitized signal including less sensitivity to change, pre-determined level of accuracy, better dynamic range, applicability to non-linear control, predictability and repeatability, insensitivity to environmental variations, replicatability, flexibility, multiplex ability and economy.
Electrosurgical units put out analog signals as their output. Processors or computers are arranged to consider digital signals and although analog to digital signals conversion is necessary, the manner in which the conversion is made bears strongly on the accuracy and ability, i.e. response time, of the circuit used.
Described herein are an electrosurgical unit control responsive to load and its method of use neither found in the literature nor practiced in the field. The literature is of interest for its teachings of the knowledge of skilled artisans at the time of this invention.